In a laser element such as a semiconductor laser used in an optical communications system or a solid-state laser used in a laser processing system, when light reflected by an optical surface or a work surface outside a laser resonator returns to the laser element, the laser oscillation is destabilized. The destabilized laser oscillation causes noise in a signal in an optical communications system, or may destroy the laser element in a laser processing system. Accordingly, an optical isolator is used to block such reflected return light, preventing the reflected return light from returning to the laser element.
Meanwhile, there is known a polarization independent optical isolator comprising as main components as shown in FIG. 1: a pair of wedge-shaped birefringent crystal plates 1, 2; a Faraday element 3 provided between the wedge-shaped birefringent crystal plates 1, 2 and made of a paramagnetic body having a rectangular cross section; and a permanent magnet 5 (see Patent Document 1). Note that a solid line in FIG. 1 shows how laser light travels in a forward direction.
Moreover, in this type of polarization independent optical isolator, for example, laser light (incident light) in the forward direction having been emitted from a laser element (not shown) and passed through a lens 4 enters the wedge-shaped birefringent crystal plate 1, and is split into an ordinary ray and an extraordinary ray, so that the rays proceed in two optical paths, respectively, and enter the Faraday element 3. After the Faraday element 3 rotates the planes of polarization by 45°, the rays enter the wedge-shaped birefringent crystal plate 2 and exit from the optical isolator in such a state that the rays are again parallel to the laser light (incident light).
On the other hand, when laser light travels in a reserve direction (i.e., when return light travels), the light enters the wedge-shaped birefringent crystal plate 2 from an emission side of the optical isolator (the right side of FIG. 1), and follows the same path as that taken when traveling in the forward direction until the light reaches the wedge-shaped birefringent crystal plate 1. When emitted from the wedge-shaped birefringent crystal plate 1, the light travels as if bypassing the lens 4 (i.e., as shown by the broken lines). In other words, the return light is never coupled into the lens 4, and thus the optical isolator can function.
Note that, in FIG. 1, reference sign γ denotes a wedge angle of each of the wedge-shaped birefringent crystal plates 1 and 2, whereas reference sign β denotes an angle formed between the return light and the incident light.